Fix typo in readme

[peterjuras]

Thanks for taking the effort to create a PR!

If you are creating an extensive issue, you might want to open an issue with your idea first, so that you don't put a lot of effort in an PR that wouldn't be accepted ;-).

PR checklist:

• Implementation
• Added tests
• Verified that there is no significant performance drop (npm run perf)
• Updated (either in the description of this PR as markdown, or as seperate PR on the gh-pages branch. Please refer to this PR). For new functionality, at least API.md should be updated
• Added type annotations (if you are unfamiliar with typescript, untyped, :any based PR's are welcome as well

[coveralls]

Coverage remained the same at 95.717% when pulling c74540c on peterjuras:master into 0da54f8 on mobxjs:master.

[mattruby]

Thanks!